In recent years "chip carriers" have emerged as a promising high-volume device packaging technique. In its simplest form a chip carrier is a substantially square shaped, relatively thin, article having a cavity with bonding pads therein which are connected to a plurality of electrically conductive leads that extend through the chip carrier body and terminate on the underside thereof. The conductive leads may be solid conductor wire (commonly referred to as leaded) or plated leads (commonly referred to as leadless). A semiconductor circuit chip is placed in the cavity, connected to the bonding pads and a cover placed over the cavity to hermetically seal the semiconductor chip therein.
A plurality of such chip carriers may be soldered to electrically conductive bonding pads on the surface of a Printed Circuit Board (PCB). The pads and/or the leads may have predeposited amounts of solder thereon or, alternatively, a solder preform may be interposed between each pad and its associated lead. The leads, solder and the pads are then placed in contact and the solder reflowed to effect the bond.
Various techniques have been used to reflow the solder to form such a surface bond. Belt furnaces and condensation soldering presently are the most widely used techniques for simultaneously bonding one or more of chip carriers to the pads on the PCB.
When placing chip carriers on the surface of a PCB it is well known to use an aluminum or stainless steel template, with the required number of square aperatures machined therein, to maintain alignment of the chip carriers to bonding pad arrays on the PCB. The template is placed proximate the surface of the substrate and the chip carriers placed therein and the solder reflowed to form the desired bond. Such square apertures are expensive to fabricate. Additionally, when condensation soldering is used to form the bond, the hot saturated vapor has limited accessability to the bonding sites because the space between the sides of the chip carriers and the edges of the apertures is very small in order to maintain alignment between the leads and the bonding pads. Additionally, liquid condensate drainage in the vicinity of the bond site is reduced allowing a liquid film to isolate vapor from the solder being melted which slows down the heating operation.
Furthermore, it is most desirable to simultaneously bond chip carriers on both sides of the PCB. In order to accomplish the surface mounting of chip carriers on a PCB using a belt furnace it is necessary to pass the PCB through the furnace in a first pass to bond the carriers to one side and then turn the PCB over, place the chip carriers thereon, and make a second pass to bond the remaining carriers. Such a two-step approach is time consuming, expensive and reduces the strength of the solder bond which was heated twice causing dissolution of a portion of the substrate metallization into the molten solder. Heretofore, condensation soldering has only been effective for bonding the chip carriers to one side of the PCB.
Accordingly, there is a need for a method and apparatus for orienting and aligning chip carrier leads to bonding pads on both upper and lower surfaces of a PCB during reflow soldering. Further, there is a need for a technique for simultaneously surface bonding of chip carriers to both sides of a PCB.